Experimental Study on Damage and Control Methods of Fracturing Fluid Retention to Tight Shale Matrix

The tight shale matrix has the structural characteristics of low porosity and low permeability. It is easy to cause water sensitivity damage, water lock damage and solid phase damage during fracturing, which greatly affects the gas reservoir transportation process of core. At the same time, fracturing fluid will invade the reservoir matrix, causing permeability damage and reducing gas production efficiency. This study analyzes the process and mechanism of fracturing fluid damage to shale matrix in the process of fracturing fluid retention, and proposes fracturing fluid damage control methods. Taking a tight sandstone reservoir in the ZJ block in South Sichuan as the research object, the mineral type, viscosity content and various physical parameters of shale gas reservoir are analyzed, and the quantitative index of fracturing fluid damage index is calculated. Using HPG as the precursor fluid, KW-1 and KDF as the drainage aids to prepare the fracturing fluid for experiment, the viscosity of the gel breaker reached 1.3 mPa·s, the interfacial tension between the gel breaker and kerosene reached 1.05 mN/m, and the surface tension was 22.8 mN/m. The fracturing fluid has good flowback performance. By collecting 4 core samples from ZJ block, the gas permeability of core samples is selected as three permeability sections 0.05·10^–3 μm², 0.15·10^–3 μm² and 0.25·10^–3 μm². And the correlation experiments of water sensitive damage, water lock damage and solid damage are carried out. The results show that when the permeability of the fracturing fluid decreases from 0.25·10^–3 μm² to 0.05·10^–3 μm², the damage value of the permeability section of the JS experimental group also increases from 8.25% to 18.35%, the movable water retention also increases from 0.032 PV to 0.046 PV, and the bound water increase increases from 0.032 PV to 0.086 PV. Therefore, the smaller the osmotic pressure is, the greater the retained amount of movable water and the increased amount of bound water are, and the greater the damage value of fracturing fluid is. In addition, when the mass fraction of XJHX in this experiment reaches 0.8%, its anti-swelling rate can reach 85%, which has excellent anti-swelling performance and can effectively reduce the permeability damage caused by fracturing fluid to shale formation.